Field of the Invention
The invention concerns to a method for determining a B1 field map in a magnetic resonance tomography scanner, the map describing the local field strength of a radiofrequency electromagnetic alternating B1 field radiated by a transmit antenna to excite nuclear spins in at least one measurement region of a subject, wherein, as part of a measurement sequence, the transmit antenna radiates multiple excitation pulses that change the magnetization of an excitation region encompassing the measurement region according to an assigned flip angle, and a first measured value is acquired in a first measuring interval and a second measured value is acquired in a second measuring interval by a reception antenna, the measured values relating to the magnetization in the measurement region, wherein a provisional flip angle value for the flip angle assigned to a selected excitation pulse in the measurement region is determined as a function of the first and second measured values. The invention also concerns a magnetic resonance tomography scanner for implementing such a method.
Description of the Prior Art
The field strength of the RF field used in magnetic resonance tomography scanners to excite slices of an object under examination, the so-called B1 field, is usually not homogeneous throughout the measurement region. Inhomogeneities may be caused by the system geometry of the magnetic resonance tomography scanner and by the object under examination itself. By taking this inhomogeneity into account during data analysis, the measurement quality of many measuring methods, e.g. of T1 imaging techniques, can be significantly improved. B1 inhomogeneity acquisition can also be used for calibration or fault diagnostics in magnetic resonance tomography scanners or for pulse calculation in multichannel operation. It is therefore known to measure B1 field maps that describe a local strength of the B1 field in different measurement regions, particularly in the form of a two-dimensional pixel or three-dimensional voxel field.
The local B1 field strength can be acquired by radiating an excitation pulse having a predefined flip angle, and measuring the flip angle achieved locally by that excitation pulse. The problem here is that when individual slices are excited, the flip angle actually achieved varies across the slice thickness even if the B1 field is completely homogeneous. In the case of magnetic resonance tomography scanners, slice selection is performed by choosing an appropriate excitation pulse which, because of its frequency spectrum and a slice selection gradient, excites the object under examination in a slice-specific manner. However, for homogeneous excitation of a defined slice, the excitation pulse would have to represent a Fourier transform of a square wave function in the frequency domain, i.e. a sinc function. However, a sinc excitation pulse would be of infinite length. Sinc pulses that are time-limited by multiplying them by a so-called window function can be used as actual excitation pulses. However, depending on the specifically selected window function, this results in amplitude ripple in the frequency domain and/or in amplitude drop-off toward higher and lower frequencies, i.e. to a “rounding” of the square wave function in the frequency domain. Accordingly, the flip angle achieved varies locally in the case of excitation using such an excitation pulse, wherein it oscillates in the direction of the slice selection gradient and/or decreases from the center of an excitation slice outward to the edges. However, in the case of a magnetic resonance tomography scanner, the magnetization is always acquired as an averaging over an excited slice. A measured flip angle therefore differs from a flip angle in the center of the excitation region which is usually to be determined, wherein the error depends on the pulse shape used.
To avoid this problem, a number of approaches are known in the prior art. It is possible to use preparation pulses, for which a flip angle is to be determined, that are not slice-selective or that have a large slice width compared to the other measurement pulses used in the sequence. However, in this case it is not possible to measure multiple adjacent slices in rapid succession, as these would be affected by preceding non-slice-selective excitations.
More homogeneous excitation for the same slice width can be achieved if preparation pulses having a high bandwidth time product are used. However, corresponding pulse shapes result in large RF pulse amplitudes. If large flip angles are to be simultaneously achieved, the bandwidth time product may be technically limited.
The described inhomogeneities can be prevented from affecting the flip angles acquired if phase-based methods are used for determining flip angles. However, these methods are prone to interference from inhomogeneities in the main magnetic field, may have long measurement times and/or limited dynamic ranges and can result in high SAR loads. It is therefore desirable to use amplitude-based methods for determining flip angles and therefore B1 maps.